1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to a charger included in an image forming apparatus and free from irregular charging.
2. Description of the Background Art
Generally, a copier, printer, facsimile apparatus, printer or similar image forming apparatus includes a charger for uniformly charging the surface of a photoconductive element or similar image carrier. A latent image is formed on the charged surface of the image carrier in accordance with image data and then developed to become a toner image. The toner image is transferred from the image carrier to a sheet or recording medium. A fixing unit fixes the toner image on the sheet to thereby produce a print.
A non-contact type of charging system using the corona discharge of a charger is one of conventional charging systems. The problem with this type of charging system is that it produces discharge products including ozone and nitrogen oxides. Ozone with high density staying in the image forming apparatus oxidizes the surface of the image carrier and thereby lowers the sensitivity and chargeability of the image carrier, resulting in low image quality, as reported in the past. Further, ozone accelerates the deterioration of other various members as well and thereby reduces the service lives thereof.
On the other hand, nitrogen oxides resulting from discharge react to moisture present in the air to thereby produce nitric acid and react to, e.g., metal to thereby produce a metal nitrate. While such products have high resistance in a low humidity environment, the resistance decreases in a high humidity environment because the products react to moisture present in the air. A thin film of nitric acid or that of metal nitrate formed on the image carrier makes an image blurred or otherwise defective. This is because the resistance of the above film decreases due to the adsorption of moisture to thereby destroy a toner image formed on the image carrier.
Moreover, nitrogen oxides stay without being decomposed in the air. As a result, compounds derived from nitrogen oxides continuously deposit on the image carrier even when charging is not effected, i.e., even during an interval between consecutive processes. The compounds penetrate into the image carrier little by little as the time elapses, constituting one of major causes of deterioration of the image carrier.
It has been customary to remove deposits from the image carrier by shaving the element little by little during cleaning. This kind of scheme, however, increases the cost and brings about deterioration due to aging.
Another conventional charging system is a contact type of charging system in which a charge roller, a charge brush or similar charging member contacts or adjoins, but does not contact, the image carrier. This type of charging system is disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 57-178257, 56-104351, 58-40566, 58139156, 58-150975, 63-7380 and 2000-242064. The contact type of charging system is contrastive to the non-contact type of charging system in that it produces a minimum of ozone, reduces the power supply cost because of low required voltage, and facilitates the design of electric insulation. Of course, the contact type of charging system is free from the problems ascribable to nitrogen oxides.
More specifically, in the contact type of charging system, the charger charges the image carrier on the basis of the migration of charge when contacting the element or charges it on the basis of gaseous discharge when adjoining it. During charging, a voltage higher than one applied at the beginning of charging is applied to the image carrier. Also, an AC voltage is superposed on a DC voltage corresponding to a target charge potential, so that an oscillation voltage is applied to the photoconductive element. The AC voltage obviates an irregular potential distribution ascribable to resistance and instability during charging that the DC voltage cannot cope with alone. Japanese Patent Laid-Open Publication No. 8-106200, for example, describes charging of the kind using charge migration specifically. Japanese Patent Laid-Open Publication No. 63-149669, for example describes charging of the kind using oscillation voltage in detail.
In the contact type of charging system using a charge roller as a conductive member, the charge roller is, in many cases, made up of a conductive core and conductive rubber covering the surface of the core as a charge supply layer. Conductive rubber, however, brings about the following problems. Assume that the image forming apparatus is not operated over a long time with the charge roller contacting the image carrier. Then, part of the conductive rubber elastically deformed in contact with the image carrier permanently deforms. As a result, when the charge roller is rotated later, it cannot uniformly charge the image carrier by charge migration because its diameter is not uniform. In addition, rubber itself is highly water-absorptive and therefore noticeably varies in resistance in accordance with the environment, further obstructing uniform charging.
Apart from the deformation and water absorption of rubber, the composition of rubber itself sometimes obstructs uniform charging, as will be described hereinafter. Rubber needs some different kinds of plasticizers and activators for exhibiting elasticity and coping with deterioration. In addition, a dispersion promoting agent is sometimes added to rubber in order to disperse a conductive pigment. On the other hand, the surface of the image carrier is, in many cases, formed of polycarbonate resin, acrylic resin or similar amorphous resin. The amorphous resin is easily influenced by the plasticizers, activators and dispersion promoting agent contained in rubber, making it difficult to uniformly charge the image carrier. Moreover, impurities are apt to enter the nip between the image carrier and the charge roller and contaminate the charge roller, resulting in defective charging. Particularly, when the image carrier and charge roller are held in contact over a long time, the impurities contaminate the image carrier and are likely to produce horizontal stripes or similar defects in an image.
The charge roller adjoining, but not contacting, the image carrier solves the problems with the charge roller contacting the same discussed above. However, even the charge roller adjoining the image carrier has the following problems left unsolved. Generally, in a developer used for development, silica is coated on the surfaces of toner grains in order to enhance efficient image transfer and image quality and to stabilize image quality without regard to the number of times of image formation repeated. Although toner is expected to be transferred to a sheet during image transfer, it is not fully transferred, but partly remains on the image carrier.
An image forming cycle includes a cleaning step for removing the toner left on the image carrier after image transfer as well as paper dust and other impurities deposited on the image carrier during image transfer. The cleaning step, however, sometimes fails to remove silica coated on the toner grains and paper dust because they are far small in size than the toner grains. When part of the image carrier moved away from a cleaning station arrives at the charger, silica and paper dust deposit on the charge roller adjoining the image carrier and thereby obstruct uniform charging. Particularly, an electrostatic force derived from gaseous discharge aggravates the deposition of silica and paper dust on the charge roller.
The phenomenon described above is more conspicuous when the AC-biased DC voltage is applied than when the DC voltage is applied alone, as known in the art. This is because a medium resistance layer included in the charge roller discharges a far greater number of times in the case of the AC-biased DC voltage than in the case of the DC voltage. Particularly, a precondition with the charge roller is that the medium resistance layer controls the energy of a single fine discharge for thereby effecting uniform charging. However, when silica and paper dust cover the entire charge roller, the medium resistance layer fails to exhibit its function and causes discharge with great energy to occur, resulting in a scale-like irregular charge pattern.
I conducted a series of experiments relating to the above phenomenon and obtained the following results. For experiments, use was made of a modified version of a copier imagio 450 (trade name) available from RICOH CO., LTD. A bias was implemented as a DC voltage (identical with a charge potential) biased by an AC voltage (constant voltage controlled). An image forming cycle was repeated with the copier in a condition wherein a cleaning device was mounted and a condition wherein it was removed to unable cleaning.
When the cleaning device was absent on the copier, the surface of a charge roller partly whitened. The resistance of the whitened portion was higher than the resistance of a fresh charge roller by a range of about one figure. When the whitened portion was shaved off and analyzed, substances presumably derived from silica and paper dust were found and included Si atoms and Ca atoms. This indicated that silica and paper dust deposited on the whitened portion of the charge roller. As the image forming cycle was further repeated, toner deposited on the background of an image although it was small in amount. At last, irregularity presumably ascribable to irregular charging occurred in the halftone portion of an image.
A fresh charge roller and the charge roller held in the above conditions were compared as to the flow of a current. In the case of the charge roller in the condition wherein cleaning could not be effected, a current flew through the roller in the form of pulses and caused abnormal discharge entirely different from uniform charge to occur; the absolute value of the peak crest was sometimes as great as 0.7 mA. Such a current therefore failed to uniformly charge the surface of an image carrier. This result of comparison will be described more specifically later.
While a block of sponge or a brush may be used to remove silica and paper dust from the charge roller, it cannot remove the entire silica and paper dust whose grain sizes are extremely small.
Silica and paper dust deposited on the charge roller cause the surface resistance of the charge roller to vary and particularly increase the apparent resistance of the roller because they are electric resistors. It follows that when a charge potential on the surface of an image carrier is set, the voltage to be applied to the charge roller must be higher than the usual voltage. Particularly, it is necessary to raise the bias voltage in the case of the DC voltage or to increase the amplitude of the AC component in the case of the AC-biased DC voltage. This aggravates power consumption and makes bias control sophisticated. Further, even if the characteristic relating to bias voltage is changed, periodic irregular charging occurs in the portion of the charge roller where silica and paper dust are present, preventing the image carrier from being uniformly charged. Moreover, the high apparent resistance on the surface of the charge roller shifts the resistance variation dependent on the environment toward the high resistance side, increasing the change of the bias required to absorb the environment-dependent variation. This also requires a power supply assigned to the charge roller to have a sufficient margin and therefore increases the cost of the image forming apparatus.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-199604, 8-62948 and 10-288881.
It is an object of the present invention to provide a charger capable of obviating, when including a charging member implemented as a roller, defective charging ascribable to silica, paper dust and other impurities transferred from an image carrier, sophisticated control and an increase in cost, and an image forming apparatus using the same.
A charger of the present invention includes a charging member contacting or adjoining an image carrier for charging the image carrier. A conductive facing member faces a portion of the charging member other than a portion of the same that faces the image carrier. Discharge occurs between the charging member and the facing member to thereby remove impurities deposited on the charging member.
An image forming apparatus including the charger is also disclosed.
Further, a method of charging an image carrier of the present invention includes the steps of causing a charging member to contact or adjoin the image carrier, causing a facing member to face a portion of the charging member other than a portion of the same that faces the image carrier, and causing discharge to occur between the charging member and the facing member for thereby removing impurities deposited on the charging member.